Metal pretreatment and coating process

ABSTRACT

A process of coating the surface of zinc-based metals with a paint film, to give a coating of high adhesion to the metal. The surface is successively drenched in an aqueous alkaline conditioning liquid, water rinsed, zinc phosphate &#39;&#39;conversion&#39;&#39; coated and finally coated with a paint film by electrophoretic deposition. The conditioning liquid comprises a source of iron ions, an iron sequestering agent and sodium or potassium hydroxide, the pH of the liquid being at least 13.

United States Patent 1 1 3,620,949

[72] Inventors Alexander Robley Morrison [50] Field of Search A v 1 v v 1 1 204/ l 81 Caulfield; Heinz Dieter Herrmann, Boronia, both of [56] References Cied Australia UNITED STATES PATENTS 1 pp 20316 3,420,762 1/1969 Shaw ct al 204/181 Flled 16,1970 $544,440 12/1970 Weigel 204/1 8| [45] Patented Nov. 16, 1971 (73] Assignee Blm Pims Limited Primary Examiner-Howard S. W1ll1ams Melbourne Victoria Australia Attorney-Cushman, Darby & C ushman [32] Priority Apr. 11, 1969 [33] Australia [3] 53380 [54) METAL PRETREATMENT AND COATING PROCESS 5 Claims, No Drawings [52] U.S. Cl 204/l8l [51] lnt.Cl 1. 801k 5/02, C23b l3/O0 ABSTRACT: A process of coating the surface of zinc-based metals with a paint film, to give a coating of high adhesion to the metal. The surface is successively drenched in an aqueous alkaline conditioning liquid. water rinsed, zinc phosphate conversion coated and finally coated with a paint film by electrophoretic deposition. The conditioning liquid comprises a source of iron ions an iron sequestering agent and sodium or potassium hydroxide. the pH ofthe liquid being at least l3.

METAL PRE'I'REATMENT AND COATING PROCESS This invention relates to a process of pretreating and painting the surface of zinc-based metals.

By zinc-based metals we mean zinc itself and alloys of zinc with minor proportions of other alloying materials, together with those impurities known to the art to occur therein. For example the metal may be zinc sheet, formed zinc strip or zinc-based die-casting alloys, typically containing 90 percent or more by weight of zinc. Alternatively the metal surface to be treated may be a zinc coating applied to an iron or steel substrated, for example by hot dipping or electrodeposition and usually referred to as galvanized iron or steel. The zinc surface may also be passivated by, for example, subjecting it to the action of an aqueous solution of certain chromium salts, to further increase its corrosion resistance.

Metal surfaces of the above type are frequently found to provide unfavorable surfaces on which to deposit paint films, a commonly observed defect being that of poor adhesion between the paint and the metal substrate.

It has been proposed to pretreat the surface of zinc-based metals prior to painting them by subjecting the surface to the action of a chemical solution which deposits an adherent crystalline zinc phosphate conversion coating thereon. The process usually involves a precleaning stage and optionally a water rinse to remove dirt and/or oily residues from the surface prior to its chemical treatment. The phosphate coating is believed to have a beneficial keying action on paint coatings applied to its surface, thus improving the paint to metal adhesion. Although this treatment has given satisfactory results in many instances, there still exist circumstances in which the adhesion between the metal surface and the paint coating may fall short of acceptable commercial standards.

We have now found that paint films of surprisingly strong adhesion can be applied to zinc-based metals by a multistage process in which the metal is drenched in an aqueous conditioning bath as hereinunder defined, water rinsed conversion coated with a zinc phosphate conversion coating and then a paint film applied to the surface by electrophoretic deposition.

According to the present invention we provide a process of coating the surface of zinc-based metal by successively a. drenching the surface of the metal with an aqueous conditioning liquid comprising a solution in water of 1.5 to 10.0 percent by weight of sodium or potassium hydroxide together with at least 0.05 percent by weight concentration each of a source of iron ions (calculated as total added Fe) and a sequestering agent for iron at the pH of the liquid which shall be at least 13 b. rinsing aqueous conditioning liquid from the metal with water c. depositing a zinc phosphate conversion coating on the metal surface and d. electrophoretically depositing a paint film thereon.

In carrying out the process, the metal to be treated is first drenched with the aqueous conditioning liquid which is applied to it by, for example, spraying, flow-coating or a dipping process. Small areas of metal can be drenched manually using an absorbent pad saturated with the conditioning liquid. When the drenching is carried out in an industrial dipping tank, the metal may be given a preliminary cleaning in a suitable bath before drenching to avoid buildup of deposits in the dipping tank, although it is not essential to do so. As drenching proceeds, the surface of the metal takes on a dark grey to black appearance and drenching is judged to be completed when a color change has been observed over the whole surface of the metal, usually within 2-5 minutes. The color change need not be uniform; some areas may appear to become darker than others, possibly due to a nonuniform composition or physical state of the untreated metal surface. The conditioning liquid may be heated to accelerate the drenching process.

The conditioned metal is then rinsed with water to remove conditioning liquid and treated by a conventional zinc phosphate conversion coating process. Zinc phosphate conversion coating processes are well known and consist essentially of aqueous treatment baths in which the metal to be treated is subjected to reaction with solutions of zinc phosphates, usually in the presence of accelerators and optionally grain refining compounds. it is usual for the process to be carried out in a number of stages including, for example, prerinse and final rinse stages.

The metal is then electrophoretically coated with the desired paint.

The coating of an electrically conductive article by electrophoretic deposition in a well-known process. A conductive article as one electrode is immersed in an aqueous bath of coating composition and an electric current passed between the article and a counterelectrode in electrical contact with the bath until a desired coating is produced on the article. The deposited coating is conventionally dried or baked to produce a hard, adherent coating on the article. In current practice it is most usual for the article which is to be coated to be made the anode in the electrical circuit, the counterelectrode being the cathode and in these circumstances the coating composition will normally contain a polycarboxylic acid resin as a binder for the coating to be electrophoretically deposited. Typically, the polycarboxylic acid resin is solubilized or stably dispersed in the aqueous bath by at least partially neutralizing it with a base, e.g. ammonia, a water-soluble amine, sodium hydroxide or potassium hydroxide. For the purpose of the process of the present invention the composition of the polycarboxylic acid resin is not critical, but to achieve consistently good results we have found it preferable for the carboxylic acid resin to have an acid value in the range of 30-200 mg. KOH per gm. and for 30-100 percent of the carboxylic acid groups to be neutralized in the coating bath by a water-soluble base.

A particular advantage of our process is that it permits a tightly adhering paint film to be applied to zinc-based metals which have been chromate passivated, an objective which is not readily accomplished by other means.

Within the specified limitations, the concentration of sodium or potassium hydroxide in the conditioning liquid is not critical. At lower concentrations, conditioning rates are unacceptably slow, while at higher concentrations, excessive pitting and erosion of the zinc-based metal have been observed.

Iron is provided in the conditioning liquid by adding to it at the required concentration a source of iron ions, the nature of the source being relatively unimportant. The sole requirement is that the material chosen must be able, in the presence of the liquid of pH at least 13, to provide ions of either ferrous or ferric iron in the liquid. That is the source of iron may be a soluble iron salt or a solid component suspended in the alkaline aqueous solution and from which iron in a soluble form leaches into the solution.

We have found, for example, that suitable materials are powdered iron, iron oxides and hydroxides and soluble iron salts, e.g. ferrous sulfate, ferric chloride and ferric citrate. The iron content of the conditioning liquid is calculated on the basis of the total iron (calculated as Fe) added to the alkaline aqueous liquid and not on the concentration of iron which passes into solution. Preferably, to avoid unnecessary sludging up of the liquid, the concentration of iron added to the liquid should not exceed 2.5 percent by weight. Bearing in mind the requirement that the conditioning liquid must have a pH of at least 13, we have found that provided the iron is not introduced into the liquid in a virtually insoluble form, sufficient ions of iron leach into the solution for the process to be operative, without it being necessary to control the lower concentration limit of iron ions actually present in the liquid.

The sequestering agent is chosen from those materials known to sequester iron in the presence of water at a pH of at least l3. Thus suitable sequestering agents are certain hydroxy acids, for example gluconic, heptonic and tartaric acids and their sodium and potassium salts, and triethanolamine. Other useful sequestering agents are certain hexitols, for example sorbitol and mannitol, although they tend to be less reliable than the above agents within the stipulated pH range and must usually be employed in combination with e.g. ethylenediamine tetraacetic acid to maintain their effectiveness for prolonged working periods.

The conditioning liquid should preferably contain a slight excess, e.g. -10 percent by weight, of sequestering agent over the concentration of soluble iron in the liquid; which is readily determined by standard analytical techniques.

Additionally, the conditioning liquid may comprise anionic and/or nonionic surface active agents to promote rapid and even wetting of the surface of the metal to be treated.

The drenching action is demonstrably operative at all concentrations within our specified limits, but if the initial concentration of sodium or potassium hydroxide is very low, drenching may be somewhat sluggish. Although drenching will take place slowly at sodium or potassium hydroxide concentrations as low as 0.5 percent by weight of the conditioning liquid, the rate of drenching is slower than is desirable for a commercial process and we prefer to limit the concentration of the hydroxide to a minimum of 1.5 percent by weight. in general, drenching rates are improved by increasing the temperature of the liquid, typically to l50-l 80 F.

When the conditioning liquid is to be applied to the metal by manual or spray methods, we prefer for simplicity of operation to use a soluble iron salt as the source of iron. However, when drenching is carried out in a dip tank, our practical requirements are less restrictive and water-insoluble iron compounds provide cheap and satisfactory alternatives. if the drenching is carried out at such a rate that the conditioning liquid is rapidly depleted of iron ions, due to a low leaching rate of iron into the liquid, this is simply overcome by using a more readily solubilized source of iron. Also, if the initial concentration of iron and sequestering agent is very low, the liquid may quickly become deficient in these constituents. We prefer the alternative of increasing the concentration of iron and sequestering agent in the liquid to the more complicated method of making incremental additions of iron and sequestering agent to the dip tank.

The invention is illustrated by the following examples in which all parts are given by weight:

Example 1 Preparation of aqueous conditioning liquid in which the source of iron is a water-soluble iron salt.

Two conditioning liquids were prepared to the following compositions by dissolving in the water the materials listed, the sodium hydroxide being added last in each case.

The pH of the liquid in each case was greater than 13.

Example 2 Preparation of an aqueous conditioning liquid in which the source of iron is a water-insoluble iron compound.

75.0 parts of sodium hydroxide were dissolved in 1,000 parts of water and to the solution so-formed was added 45.0 parts of tartaric acid and 40.0 parts of an iron oxide of approximately 5 micron particle size and Pep, content of 96 percent by weight. The mixture was gently agitated and allowed to age for 24 hours before use.

Weight percent concentrations Sodium hydroxide 65.0 parts Iron 2.6 parts sequestering agent 4.3 para Example 3 A paint film is applied to zinc-coated panels by the process of the invention and its adhesion contrasted with that of paint films otherwise applied to similar panels.

Galvanized iron test panels (unannealed hot dip galvanized) were immersed for 3 minutes in a bath of conditioning liquid No. 2 of example 1 at room temperature and then water rinsed. The surface of the conditioned panels acquired a black matte appearance. One-half of the group was set aside as control group A. The remaining panels together with an equal number of cleaned but not conditioned panels (control group B) were then coated with a crystalline zinc phosphate conversion coating in standard commercial phosphating equipment and water rinsed.

The three groups of panels were then coated with a red oxide undercoat in a standard electrophoretic coating bath, dried and stoved. The undercoat binder was an epoxy-estertype polycarboxylic acid resin of acid value mg. KOH per g. neutralized in the coating bath to the extent of about 60 percent with caustic potash. The overall appearance of all undercoated panels was satisfactory, the paint films being hard and of uniform texture.

When the stove panels were scratched with the edge of a coin, the adhesion of the undercoat to the panels treated according to the invention was seen to be excellent, whereas the undercoat stripped readily from the panels of both control groups.

A further comparison was made of the film adhesion to the panels by cutting a grid of two sets of parallel lines set at right angles through the undercoat films, applying strips of adhesive tape to the grid and then rapidly stripping the tape from the panels. The undercoat was not afiected on the panels treated according to the invention, but onthe control panels about 40 percent of the area of undercoat film lying under the tape was removed with the tape, thus demonstrating once more the superior adhesion of a paint film to the metal treated according to the invention.

The example was repeated with the aqueous conditioning liquid No. l of example 1 and that of example 2 with similar results. When conditioning liquid No. l of example i was used, it was necessary to heat the liquid to F. and to immerse the test panels in the bath for about 5 minutes to get adequate drenching, as judged by the surface appearance of the panels.

Example 4 The effect of the pH of the conditioning liquid on the performance of the process of the invention is demonstrated.

A conditioning liquid was prepared to the general formula of liquid No. l of example l but with the sodium hydroxide content reduced to give a liquid of pH approximately 12.5 (3.0 parts of sodium hydroxide).

Unannealed hot dip galvanized panels were immersed in the conditioning liquid for 10 minutes at F., washed with water, conversion coated, electrophoretically coated and tested by the method of example 3. The adhesion of the paint film to the steel was poor and much inferior to that of panels coated according to the invention in example 3. No significant increase in adhesion of the paint was observed when the time of immersion in the conditioning liquid was extended to 30 minutes.

Example 5 Application of a paint film to zinc-coated panels by the process of the invention and in which the source of iron in the conditioning liquid is iron filings.

To 1,000 parts of water were added 75.0 parts of sodium hydroxide, 40.0 parts of tartaric acid, 5.0 parts of of gluconic acid and parts of iron filings. The mixture was gently agitated and allowed to age for 24 hours before use.

Unannealed hot dip galvanized steel panels were coated with a paint film and tested by the method described in exampie 3, using the above mixture as the conditioning liquid. The adhesion of paint to the metal on the panels treated according to the invention was markedly superior to that of the control panels.

We claim: 1. A process of coating the surface of zinc-based metal by successively a. drenching the surface of the metal with an aqueous conditioning liquid comprising a solution in water of 1.5 to 10.0 percent by weight of sodium or potassium hydroxide together with at least 0.05 percent by weight concentration each of a source of iron ions (calculated as total added Fe) and a sequestering agent for iron at the pH of the liquid which shall be at least 13 b. rinsing aqueous conditioning liquid from the metal with water c. depositing a zinc phosphate conversion coating on the metal surface and d. electrophoretically depositing a paint film thereon.

2. A process according to claim 1 in which the source of iron is at least one member selected from the group consisting of powdered iron, an iron oxide and an iron hydroxide.

3. A process according to claim I in which the source of iron is a soluble iron salt.

4. A process according to claim 1 in which the sequestering agent is at least one member selected from the group consisting of a sodium salt of any one of gluconic, heptonic and tartaric acids, a potassium salt of any one of the aforesaid acids and triethanolamine.

5. A process according to claim 1 in which the electrophoretically deposited paint film comprises a polycarboxylic acid resin binder having an acid value in the range of 30-200 mg. KOH per g. and from 30-100 percent of the carboxylic acid groups neutralized in the coating bath by a water soluble base.

l 1 i i i 

2. A process according to claim 1 in which the source of iron is at least one member selected from the group consisting of powdered iron, an iron oxide and an iron hydroxide.
 3. A process according to claim 1 in which the source of iron is a soluble iron salt.
 4. A process according to claim 1 in which the sequestering agent is at least one member selected from the group consisting of a sodium salt of any one of gluconic, heptonic and tartaric acids, a potassium salt of any one of the aforesaid acids and triethanolamine.
 5. A process according to claim 1 in which the electrophoretically deposited paint film comprises a polycarboxylic acid resin binder having an acid value in the range of 30-200 mg. KOH per g. and from 30-100 percent of the carboxylic acid groups neutralized in the coating bath by a water-soluble base. 